Delay equalizers are well known, and generally provide an increased delay at or around a resonant frequency at which a filter or the like exhibits a reduced delay, to provide compensation therefor. Delay equalizers of the split phase type are also known, and generally have two separate signal paths 180 degrees out of phase and summed together into a common load impedance.
A split phase delay equalizer has an amplitude response which exhibits a slight decrease or dip at or around the resonant frequency due to component Q loss (e.g., the quality factor of inductors and/or capacitors). An example of this dip is shown in "Group Delay Equalization in Communications Systems", Rosenfield et al, Application/Data Bulletin 175, Comstron SEG, p. 7, FIG. 15. This deviation from true all-pass characteristics (i.e., a non-flat amplitude response) can usually be compensated by slight changes in the design of the delay equalizer and/or its associated circuitry.
The 180 degree phase reversal is seldom ideal, due to imperfect components used to perform the phase inversion. This deviation from ideal phase reversal causes the amplitude response of the equalizer to exhibit something other than a simple dip at the resonant frequency. For example, instead of a second order parabolic type of dip curve, the amplitude response may exhibit a rise and then a dip analagous to a full sine wave. This more erratic third order deviation (e.g. full sine wave) is much more difficult to compensate. In equipment where small amplitude deviations are of concern, such as long haul telecommunication equipment, such an erratic deviation becomes a severe problem.